A model system for the study of transposable element-mediated recombination events has been developed and used to study transposition, transposon-induced fusion of plasmids and, independently, the breakdown of fused plasmids (cointegrates). The focus of the proposed study will be the compound transposons Tn9 and Tn5 and their flanking insertion elements IS1 and IS50. The system uses the plasmid pBR322 and an F-derived plasmid that contains no transposable elements. The advantages of this system include the ability to isolate fusion products easily in different genetic backgrounds, transfer the cointegrates to different backgrounds, and study in detail the recombination events leading to the fission of cointegrates. The system thereby provides a model for the study of transposon-mediated deletion and precise excision. My preliminary studies with Tn9 and Tn5 have established that their flanking insertion elements, as well as the entire transposon, can promote the formation of stable cointegrates. While very stable, the cointegrates can dissociate by various recombination events. The goals of the proposed research are to a) understand the pathways of transposition of these elements b) understand the basis of the independent action of IS1 and IS50 and the collective action of Tn9 and Tn5, c) to understand the molecular basis for the apparent differences in activities of different alleles of IS1 that preliminary experiments have revealed, and d) to characterize the recombination events that can dissociate cointegrates. These studies will be undertaken using, as a working hypothesis, the general model for transposition I have developed.